Components of gas turbine engines, such as blades (buckets), vanes (nozzles) and combustors, are typically formed of nickel, cobalt or iron-base superalloys characterized by desirable mechanical properties at turbine operating temperatures. Because the efficiency of a gas turbine engine is dependent on its operating temperatures, there is a demand for components, and particularly turbine blades and vanes, that are capable of withstanding higher temperatures. As the material requirements for gas turbine engine components have increased, various processing methods and alloying constituents have been used to enhance the mechanical, physical and environmental properties of components formed from superalloys. For example, turbine blades and vanes are often cast to have single-crystal (SX) or directionally-solidified (DS) microstructures, characterized by a crystal orientation or growth direction in a selected direction, are typically employed for more demanding applications.
In general, advancements in such technologies have been such that the maximum local metal temperatures of components formed from these superalloys are approaching the alloy melting temperatures. Accordingly, in terms of high temperature capability, it is generally necessary that gas turbine engine components have internal cooling passages through which cooling air is routed to lower the surface temperature of the component. Typical cooling schemes include one or more interior cooling passages having possibly a circuitous route through the airfoil section, with bleed air being forced through the cooling passages and discharged through openings at the surface of the component in order to transfer heat from the component. Considerable cooling air is often required to sufficiently lower the surface temperature of a blade or vane. However, the casting process and the cores required to form the cooling passages limit the complexity of the cooling scheme that can be formed within a component, and therefore limits the rate at which heat can be transferred to the cooling air.
In view of the above, it would be advantageous if high-temperature components such as gas turbine engine blades and vanes could be produced to have a more efficient internal cooling scheme. One such approach has been to fabricate blades and vanes with double walls which form a plenum that is supplied by cooling air through multiple cooling channels. This type of cooling scheme enables a more uniform supply of cooling air near the surface of such components, which significantly reduces surface temperatures. Techniques for fabricating double-walled blades and vanes have included the use of brazed foils, low pressure plasma spraying (LPPS), and electron beam physical vapor deposition (EBPVD). However, each of these processes has disadvantages. For example, the brazed foil technique requires forming the exterior shell of a component from an alloy that can be readily worked to form a suitably thin foil. As a result, the types of alloys that can be used to form the foil are extremely limited. Plasma sprayed shells inherently have a relatively high oxygen content and must be densified to eliminate porosity. Furthermore, the plasma spray process is inefficient in the use of powders, since much of the sprayed powder misses the deposit surface. Finally, the EBPVD process cannot be sufficiently controlled to be practical for production.
Therefore, it would be desirable if an improved method were available for mass producing high-temperature components with more efficient internal cooling schemes that include a double-walled shell structure.